a. Field of the Invention
The present invention concerns a hierarchical system for providing compressed and/or non-compressed digital and/or analog video signals to be broadcast, multicast, or transmitted point-to-point, to remotely located subscribers.
b. Description of the Prior Art
Over the past decade or two and continuing to the present, "off-the-air" communication systems, such as CATV for example, have become rather ubiquitous throughout most of the developed world and permit a wide diversity of programming and other broadcast information to be disseminated directly to subscribers. Through such systems, subscribers, typically residential customers, can select from among a number of program providers that offer far more diverse content than traditional "on-the-air" broadcast television channels. Given the richness of the available programming and its relatively affordable cost, the CATV industry has experienced significant historic growth.
Traditional cable distribution systems are entirely analog in nature. In essence, various program providers transmit their programming over distinct channels, via a satellite link or other dedicated long-haul communication paths, to a "cable headend" location. As determined by the program provider, each of these signals may be transmitted in scrambled form, particularly for so-called "premium" channels, or in non-scrambled form.
As shown in FIG. 1, equipment, such as a network feed downlink 102 and a group of integrated receiver/decoders (or "IRDs") 104, at a cable headend location receives the transmitted signals that appear on a variety of different channels, and appropriately amplifies and conditions each of these signals (not shown). Then, equipment, such as a plurality of remodulators 106 and a channel combiner 108, remodulate and frequency division multiplex each of the resulting signals into a single broadband, frequency division multiplexed signal. This broadband signal is further amplified and driven (e.g., by distribution fan-out nodes 110) onto a geographically dispersed hardwired coaxial cable network for distribution to individual subscribers 112. Each subscriber 112 utilizes a converter (also commonly referred to as a "set-top box") which terminates a so-called cable "drop" in the network. Hence, the cable television headend station is often referred to as a "community antenna."
The set-top box receives the broadband signal and, based on a subscriber selection generally entered through a remote control device, selects a given cable channel for display, demultiplexes a signal for that channel from the received broadband signal, descrambles that signal, if necessary and if authorized to do so, and applies a resulting baseband video signal to an input of a television receiver (or applies an RF input to channel 3 or 4 of the television receiver) for subsequent display and/or to an input of a video cassette recorder (VCR) for recording. Newly manufactured television receivers and VCRs possess so-called "cable-ready" tuners which provide this demultiplexing function, but which do not include descrambling circuitry.
Two fundamental technical limitations of the known CATV system have tended to limit viewer choice and hence the attractiveness of cable service; namely the limited bandwidth of coaxial cable and the lack of support for back-channel communications.
First, a traditional cable system is structured, as noted above, with a single point of signal origination (i.e., the headend) for all its subscribers. That is, a traditional cable system provides a so-called "one-to-many" (or "broadcast" or "multicast") distribution. While such a system advantageously provides a greater number of channels than does on-the-air television broadcasts, the number of cable channels is still quite restricted. As a consequence of a limited number of channels, many individual subscribers increasingly find that, at one time or another, an insufficient number of different programs on different channels are then available to satisfy their current interest.
To reiterate, in a conventional analog system, the same set of multiplexed channels is transmitted from a cable headend into a coaxial cable distribution network, through which this set of multiplexed channels is "fanned-out" (i.e., "broadcast") to all subscribers in the system. Thus, the number of channels that can be simultaneously transmitted is determined, in good measure, by the bandwidth of the physical coaxial cable used in the distribution network. Coaxial cable, as with any transmission media, has a limited bandwidth. In the context of frequency multiplexed analog video signals, coaxial cable typically has a capacity to carry approximately 50-70 analog frequency division multiplexed video channels over an appreciable distance without undue signal attenuation. These systems typically utilize a frequency band of approximately 5-400 MHz or so.
As the number of frequency division multiplexed video channels increases, the highest carrier frequency applied to the cable also increases. As transmitted cable carrier frequencies increase much above several hundred megahertz, cable losses, principally due to inductive "skin effect" as well as capacitive affects, increase substantially and, accordingly, greatly attenuate the transmitted signal at these high frequencies, thus frustrating the use of additional high frequency channels.
Broadband in-line repeaters are periodically placed in series with a distribution trunk to (e.g., at distribution or fan-out nodes 110) re-amplify and re-condition the multiplexed signal that propagates down the trunk to compensate for attenuation present in that trunk. Traditionally, one method to overcome this attenuation is to simply decrease the inter-repeater spacing in the network. However, doing so, particularly given the rather wide geographic coverage of a coaxial cable network and the number of distribution trunks used therein, often entails a substantial increase in the number of repeaters and thus, cost.
In addition to the increased cost associated with increasing the number of repeaters, the potential for adverse inter-modulation and cross-modulation products is also heightened. Such inter-modulation and cross-modulation products result from system non-linearities and the presence of numerous video carriers, and thus introduce noticeable distortion into a multiplexed cable signal. In that regard, so-called "composite triple beat" products (modulation products of the general type (2f.sub.1 .+-.f.sub.2) or (f.sub.1 +f.sub.2 .+-.f.sub.3)) can greatly degrade the performance of such a system. Coherent oscillators may be used to regenerate (and convert) carrier frequencies such that they are harmonically related thereby ameliorating these adverse modulation products somewhat. Nevertheless, adverse inter-modulation and cross-modulation products increase with increases in repeater output levels, as well as with increases in the number of repeaters cascaded along any distribution path in the cable network. The presence of these adverse inter-modulation and cross-modulation products effectively limits both the number of repeaters that can be cascaded and hence the length of an entire distribution path itself (i.e. an entire coaxial cable run from a headend location to a subscriber). If the number of channels were to increase, the repeater separation would have to be reduced to compensate for lowered output levels while avoiding adverse modulation products which, in turn, nevertheless increases system cost.
Hence, the above described phenomena intrinsic to analog co-axial cable distribution have effectively limited the number of channels simultaneously available in any conventional analog cable system to approximately 50-70. This limitation has been ameliorated somewhat, but only in newly constructed cable systems that utilize a low-loss optical fiber distribution network. Such low-loss optical fiber distribution networks possess an increased bandwidth (generally on the order of 1 GHz--two or three times that of a coaxial cable distribution network) over that of a traditional coaxial cable based distribution network. However, since most cable distribution networks have already been installed quite a few years ago, the vast majority of existing cable systems utilize a coaxial cable, rather than optical, based distribution network and are so limited to 50-70 simultaneous channels.
If, as often occurs, the subscriber is not interested in programming then being carried by any of the 50-70 cable channels, they will turn to other pursuits. If this occurs frequently enough, the subscriber, possibly out of repeated dissatisfaction or frustration, may terminate their cable service entirely.
Second, because traditional analog cable systems are unidirectional, the subscriber, even if they are interested in a particular program on any given cable channel, is relegated to a pre-defined broadcast schedule of a corresponding channel provider. Although the proliferation of VCRs has alleviated this concern by storing the channel broadcast for later viewing at a more convenient time, the inconvenience and complexity, as seen by many subscribers, in programming a VCR for recording and time-shifted viewing, also contributes to continued viewer frustration and dissatisfaction with cable systems.
The technical community serving the cable industry has grappled with these difficulties for some time but, as of yet, no effective global solution has been put forth in the art.
Continuing advances in digital storage technology and recent advances in video compression methods have led to a number of proposals for new video distribution systems. However, at the present time, each of these proposed systems includes serious drawbacks which preclude its current implementation on a large scale. The art has unequivocally recognized that digital signals are far less prone to noise, affects of attenuation and other adverse phenomena typically associated with analog signals and, through use of sophisticated error correction algorithms, can readily be corrected for artifacts and other transport errors. For those reasons, digital signals are routinely favored for transmitting information. Indeed, U.S. Pat. Nos. 5,400,401 (hereinafter referred to as "the Wasilewski et al patent"), 5,410,343 (hereinafter referred to as "the Coddington et al patent"), 5,412,416 (hereinafter referred to as "the Nemirofsky patent"), 5,442,389 (hereinafter referred to as "the Blahut et al patent"), and 5,508,732 (hereinafter referred to as "the Bottomly et al patent"), each discuss video distribution systems which provide digital video signals to subscribers. Unfortunately, the systems discussed in these patents require large capital outlays for providing or upgrading the infra-structure of a distribution system and also require relatively sophisticated and expensive set-top boxes. Moreover, although, at first blush, enhancing a cable system by merely converting the system from analog to digital signaling appears attractive, doing so would necessitate replacement of much of the embedded analog circuitry in that system. In view of the enormous investment made over the past few decades in embedded physical plant, both at cable head-ends and in subscriber set-top boxes, for analog signal distribution and reception equipment, converting, at one time, a cable system to handle end-to-end digital signaling is prohibitively expensive and hence totally impractical. Furthermore, such a "one-shot" conversion would disrupt service to many, if not all, current subscribers. Therefore, any solution aimed at enhancing utility, functionality and subscriber attractiveness of a cable system should maintain compatibility with the existing analog plant to the fullest extent possible. However, given the advantages of transmitting signals digitally, such a system should permit a gradual and non-disruptive migration towards an all digital system. Further, such a system should, ideally, be configurable to serve various types of network, whether they are, for example, analog co-axial cable networks or digital optical fiber networks.
Starting in the early 1990s and continuing to the present, significant developments are appearing in the art of communication and digital data processing that, if utilized, hold great promise in overcoming the technical limitations, as discussed above, inherent in traditional analog cable systems. Moreover, these developments, if properly utilized, should profoundly enhance the utility, functionality and attractiveness of existing cable systems to both present and potential subscribers while advantageously maintaining full compatibility with these existing analog systems. Furthermore, these developments should make other distribution systems more attractive.
Currently, extremely fast and sophisticated digital processors are becoming commercially available along with high capacity low-cost digital storage mechanisms. Processors are evolving, both in terms of their continually increasing sophistication and decreasing cost, to a level at which these such processors may cost-effectively function as video servers to provide digitized video data at a sufficiently rapid rate to permit its near real-time or even real-time viewing.
Moreover, the streaming of more than one video file is now feasible. Furthermore, large-capacity magnetic hard disk drives, as well as optical disk drives, with an individual capacity in excess of several gigabytes are now available at a price of less than U.S. $1000/drive. Such drives may be arranged into high capacity disk farms, having a storage capacity of hundreds of gigabytes or several terabytes. Such disk farms could well store digitized video libraries of one sort or another. With further advances in storage technology, the price of such drives is only likely to fall while their capacity continues to rise.
Unfortunately, however, a digitized video image, let alone full motion video, can generate a rather large data file, often in excess of several megabytes of bit-mapped data per frame. Since full motion NTSC (National Television Standards Committee) video requires 30 frames/second, an advertisement lasting 30 seconds to one minute, a program lasting 30 minutes or more, or a movie lasting 1-2 hours or more, will require a corresponding video file that has an enormous amount of data, particularly if stored in simple bit-mapped form. Storage of such a bit-mapped file could still prove costly even at current and anticipated near term costs per megabyte.
Fortunately, occurring along with the above mentioned development in processor and storage technology is the rather recent emergence not only of a rather robust and highly accurate compression/decompression algorithm for full motion video (and associated audio), but also of rather inexpensive integrated circuits (ICs) that encode and decode video data in accordance with this algorithm. This algorithm is the MPEG-2 standard promulgated by the Motion Picture Experts Group (MPEG) and jointly published by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC), in draft form on Nov. 11, 1994, as International Standard ISO/IEC 13818 (incorporated herein by reference). By exploiting the fact that high degrees of inter-frame and intra-frame redundancies are inherent in video, the MPEG-2 algorithm advantageously yields an extremely high rate of video compression. Consequently, the MPEG-2 compression algorithm drastically reduces the file size needed to store, as well as the bandwidth needed to transmit, an item of digitized full motion video.
Although the MPEG-2 standard is known to those skilled in the art, its relevant features are briefly described below for the reader's convenience. The MPEG-2 standard focuses on the encoding and transport of video and audio data. In general, the MPEG-2 standard uses compression algorithms such that video and audio data may be more efficiently stored and communicated.
The International Organisation for Standardisation (or the Organisation Internationale De Normalisation) (hereinafter referred to as "the ISO/IEC") has produced drafts of the MPEG-2 standard for the coding of moving pictures and associated audio. This standard is set forth in four documents. The document ISO/IEC 13818-1 (systems) specifies the system coding of the specification. It defines a multiplexed structure for combining audio and video data and means of representing the timing information needed to replay synchronized audio and video sequences in real-time. The document ISO/IEC 13818-2 (video) specifies the coded representation of video data and the decoding process required to reconstruct pictures. The document ISO/IEC 13818-3 (audio) specifies the coded representation of audio data and the decoding process required to reconstruct the audio data. Lastly, the document ISO/IEC 13818-4 (conformance) specifies procedures for determining the characteristics of coded bitstreams and for testing compliance with the requirements set forth in the ISO/IEC documents 13818-1, 13818-2, and 13818-3. These four documents, hereinafter referred to, collectively, as "the MPEG-2 standard" or simply "the MPEG standard", are incorporated herein by reference.
A bit stream, multiplexed in accordance with the MPEG-2 standard, is either a "transport stream" or a "program stream". Both program and transport streams are constructed from "packetized elementary stream" (or PES) packets and packets containing other necessary information. A "packetized elementary stream" (or PES) packet is a data structure used to carry "elementary stream data". An "elementary stream" is a generic term for one of (a) coded video, (b) coded audio, or (c) other coded bit streams carried in a sequence of PES packets with one and only stream identifier (or "ID"). Both program and transport streams support multiplexing of video and audio compressed streams from one program with a common time base.
Transport streams permit one or more programs with one or more independent time bases to be combined into a single stream. Transport streams are useful in instances where data storage and/or transport means are lossy or noisy. The rate of transport streams, and their constituent packetized elementary streams (PESs) may be fixed or variable. This rate is defined by values and locations of program clock reference (or PCR) fields within the transport stream.
FIG. 21 illustrates the packetizing of compressed video data 2106 of a video sequence 2102 into a stream of PES packets 2108, and then, into a stream of transport stream packets 2112. Specifically, a video sequence 2102 includes various header information 2104 and associated compressed video data 2106. The video sequence 2102 is parsed into variable length segments, each having an associated PES packet header 2110 to form a PES packet stream 2108. The PES packet stream 2108 is then parsed into segments, each of which is provided with a transport stream header 2114 to form a transport stream 2112. Each transport stream packet of the transport stream 2112 is 188 bytes in length.
Although the syntax of the transport stream 2112 and transport stream packets is described in the MPEG-2 standard, the fields of the transport stream packet pertaining to the present invention will be described below with reference to FIG. 22 for the reader's convenience. As shown in FIG. 22, a transport stream 2112 includes one or more 188 byte transport stream packets 2200, each of the transport stream packets 2200 having a header 2114 and an associated payload 2216.
Each header 2114 includes an eight (8) bit synch byte field 2218 and a thirteen (13) bit packet identification (or PID) field 2220. The synch byte field 2218 has a value of "01000111" (or 47 hex) and identifies the start of a 188 byte transport stream packet 2200. The PID field 2220 indicates the type of data (e.g., audio, video, secondary audio program (or "SAP"), private, etc.) stored in the payload 2216 of the 188 byte transport stream packet. Throughout this application, the term "private data" is often used to refer to a transport stream packet 2200 having a PID 2220 value which identifies data in the payload 2216 of the transport stream packet 2200 as proprietary data, for special use (e.g., on-screen displays) by the system of the present invention. In such instances, "private data" should not be confused with private data which may be carried in the transport private data field 2250 of the transport stream header 2114, which may also be used to carry proprietary data. In any case, the meaning of "private data" will be apparent based on the context in which it is used. Certain PID values are reserved.
The payloads 2216 of one or more transport stream packets 2200 may carry "packetized elementary stream" (or PES) packets 2300. To reiterate, a "packetized elementary stream" (or PES) packet 2300 is a data structure used to carry "elementary stream data" and an "elementary stream" is a generic term for one of (a) coded video, (b) coded audio, or (c) other coded bit streams carried in a sequence of PES packets with one and only stream ID.
FIG. 23 is a diagram which illustrates the syntax of a PES packet 2300. As FIG. 23 shows, a PES packet 2300 includes a 24 bit start code prefix field 2302, an eight (8) bit stream identifier field 2304, a sixteen (16) bit PES packet length field 2306, an optional PES header 2308, and a payload section 2106. Each of these fields is described in the MPEG-2 standard. However, for the reader's convenience, the fields particularly relevant to the present invention are described below.
The sixteen (16) bit PES packet length field 2306 specifies the number of bytes in the PES packet 2300 following this field 2306. A value of 0 in this field 2306 indicates that the PES packet length is neither specified nor bounded. Such an unspecified and unbounded PES packet 2300 is only allowed in PES packets whose payload is a video elementary stream contained in transport stream packets. As can be deduced from the description of the PES packet length field 2306, the PES packet 2300 can be much longer (e.g., 4000 bytes) than the length of the payload 2216 of a 188 byte transport stream packet. Thus, as shown in FIG. 1, a PES packet 2300 is typically carried in consecutive payloads 2216 of a series of transport stream packets 2200. The payload 2106 of a PES packet 2300 may carry a sequence of video frames or audio frames, for example.
FIG. 24 is a block schematic showing the steps of encoding, communicating (from location 2440 to location 2450), and decoding video and audio data in accordance with the MPEG-2 standard.
As shown in FIG. 24, at a first location 2440, video data is provided to a video encoder 2402 which encodes the video data in accordance with the MPEG-2 standard (specified in the document ISO/IEC 13818-2 (video), which is incorporated herein by reference). The video encoder 2402 provides encoded video 2404 to a packetizer 2406 which packetizes the encoded video 2404. The packetized encoded video 2408 provided by the packetizer 2406 is then provided to a first input of at least one of a program stream multiplexer 2410 and a transport stream multiplexer 2412. For the purposes of understanding the present invention, it can be assumed that program streams are not generated.
Similarly, at the first location 2440, audio data is provided to an audio encoder 2414 which encodes the audio data in accordance with the MPEG-2 standard (specified in the document ISO/IEC 13818-3 (audio), which is incorporated herein by reference). The audio encoder 2414 provides encoded audio 2416 to a packetizer 2418 which packetizes the encoded audio 2416. The packetized encoded audio 2420 provided by the packetizer 2418 is then provided to a second input of at least one of the program stream multiplexer 2410 and the transport stream multiplexer 2412.
The transport stream multiplexer 2412 multiplexes the encoded audio and video packets and transmits the resulting multiplexed stream to a second location 2450 via communications means 2422. At the second location 2450, on a remote end of the communications means 2422, a transport stream demultiplexer 2424 receives the multiplexed transport stream. Based on the packet identification (or PID) number 2314 of a particular packet, the transport stream demultiplexer 2424 separates the encoded audio and video packets and provides the video packets to a video decoder 2430 via link 2428 and the audio packets to an audio decoder 2434 via link 2432. The transport stream demultiplexer 2424 also provides timing information to a clock control unit 2426. The clock control unit 2426 provides timing outputs to the both the video decoder 2430 and the audio decoder 2434 based on the timing information provided by the transport stream demultiplexer 2424. The video decoder 2430 provides video data which corresponds to the video data originally provided to the video encoder 2402. Similarly, the audio decoder 2434 provides audio data which corresponds to the audio data originally provided to the audio encoder 2414.
This ability to compress video has led some to think about video distribution systems in which a number of full or feature length movies are actually stored on extremely large DRAMs in addition to, or instead of, storage on large magnetic or optical disk drives or on magnetic tape drives. For example, U.S. Pat. No. 5,410,343 (hereinafter referred to as "the Coddington et al patent") discusses a video distribution system having a DRAM storing 15 to 25 compressed feature length movies, each occupying approximately 1.2 gigabytes. Similarly, U.S. Pat. No. 5,442,389 (hereinafter referred to as "the Blahut et al patent") discusses a video distribution system having a 206 gigabyte memory for storing over 150 full-length movies. Unfortunately, however, DRAM fabrication and storage densities have not yet advanced to the point to which systems employing such large DRAMs are feasible economically.
All digital video distribution systems which utilize the public switched telephone network (or PSTN) have also been proposed. For example, the Coddington et al patent uses an asymmetrical digital service line (ADSL) interface over a twisted pair which is frequency division multiplexed into a 4 KHz wide voice channel, an 8 Kbps reverse digital channel centered on 95 KHz, and a 1.6 Mbps video channel from 100 to 500 KHz. Similarly, U.S. Pat. No. 5,508,732 (hereinafter referred to as "the Bottomley et al patent") uses a T1 (i.e., a 1.544 Mbps) link to a subscriber to provide near video-on-demand (or NVOD). Unfortunately, the telephone system was designed to handle voice traffic (for example, 8 bit samples at 8 Khz, or 64 Kbps voice traffic) for relatively short periods of time. For example, in a typical residential area, central offices are designed based on the expectation that an average telephone customer uses the network for 3 CCs (i.e., 3 hundred call seconds) or 5 minutes per hour. The telephone system, at this point, is not designed to handle 1.544 Mbps video signals on T1 links (equivalent to 24, 64 Kbps voice channels) occupying a line for 90 minutes without interruption. Thus, substantial capital expenditures are required before the telephone network can handle the substantial additional traffic introduced by such video delivery and video-on-demand services.
Thus, although emerging new digital technology, particularly in the areas of storage density and video compression, appears to provide several sorely needed building blocks for a video delivery system, proposed systems require substantial capital expenditures to improve existing distribution networks and to provide set-top boxes with sophisticated decoding and formatting chipsets. Hence, the proposed systems offer an "all or nothing" approach that requires a capital expenditure that is, frankly, unpalatable to many in the video distribution industry.
Thus, a substantial and crucial obstacle remains: integrating these emerging new digital technologies with existing video distribution networks, such as analog cable systems, in a way that maintains full compatibility with and thereby preserves, to the fullest extent possible, the existing investment made in all the existing analog physical plant, i.e., all the existing analog set-top boxes and head-end analog signal distribution equipment.
Ideally, the infusion of this technology into a cable system should occur in a manner that not only provides enhanced cable system functionality and increased viewer choice and satisfaction but also entails relatively minor changes, if any, to the existing analog physical plant.
In that regard and even with continued use of analog program signaling to the subscriber, we expect that a flexible digital video file server system could spawn a wide variety of new services heretofore not available on traditional analog cable systems. For example, one new class of cable services, collectively referred to herein as "targeted programming", could include the provision and selective distribution of customized video programming matched to the needs of a specific target audience. Different programming would be simultaneously provided, over the same cable system, to different subscriber audiences thereon.
Such targeted programming might be advertisements of high interest to, and directed to, a certain demographic and/or geographically located class of individuals--as pre-defined by an advertiser. Such targeted programming might include information, such as, e.g., a textual schedule of local events with accompanying "video clips" thereof, that is to be distributed to a local community of potential attendees and/or other interested parties that are presumed to be interested--all as predefined by an event promoter. For example, the delivery of a channel having video clips of nationally released movies with an overlay or on-screen-display (or OSD) of local theaters and play-times of the movie would be attractive. In each instance, the ensuing recipients would be much smaller than all the cable subscribers at large on the system. Hence, a system having multicasting, as well as broadcasting, functionality is needed.
Another new class of cable services, collectively referred to herein as "subscriber-initiated programming" or "requested programming", could be based on so-called "near video-on-demand" (or NVOD) through which each subscriber could choose specific programming for delivery by the cable system. Here, that subscriber would be allowed, through pre-defined interactivity with the video provider, to request, from a large archival library, just what video material, e.g. a particular movie, television show or documentary, that particular subscriber wants to view. The server would then inform the subscriber of the time and channel on which that material will be played. The cable system, in conjunction with the server, would then retrieve and play the material but with only a minimal, if any, delay from the time at which the subscriber entered his or her request. If this material is to be provided in a scrambled form, the server would provide appropriate commands to authorize the set-top box serving that subscriber to de-scramble the incoming signal as required for proper viewing of the requested material.
We anticipate that by providing targeted programming, whether it is relevant advertisements or other useful information distributed to specific cable subscribers along geographic and/or demographic boundaries, or subscriber-initiated (or requested) programming, such as entertainment, or even some combination of the two, cable systems can sharply increase their attractiveness to current and future subscribers with, concomitantly, the cable industry, as a whole, eventually experiencing sharply higher market penetration and revenue growth.
Thus, a need now exists in the art for a cost-effective digital video file server system that can be readily integrated, with minimal disruption, if any, into existing analog cable systems to greatly enhance the functionality and subscriber attractiveness of such systems. Such a file server system should preserve, to the extent possible, all the existing analog physical plant, thereby maintaining compatibility with existing analog signal distribution equipment and subscriber set-top boxes currently in use. Furthermore, such a file server system should provide archival storage of a substantial amount of video material and, for purposes of economic implementation, utilize video compression, particularly, though not exclusively, the MPEG-2 standard. To enhance subscriber attractiveness, such a video file server, when integrated into an existing cable system, should be able to provide services, such as targeted programming and subscriber-initiated (or requested) programming, currently not available on conventional analog cable systems.
The advantages of digitally transmitting any data, including video data, cannot be forgotten however. Although, as discussed above, the capital expenditures for providing an all digital video distribution network are indeed staggering, we expect that such networks will, inevitably, be built. Moreover, although prohibitively expensive now, we believe that the cost of sophisticated set-top boxes will continue to decrease and will inevitably reach a point of being economically attractive. Thus, the system of the present invention must be flexible to permit a migration to all digital distribution networks.